1. Field of the Invention
The present invention relates to a method for injection molding an uncrystallized or crystallized polyethylene terephthalate into a desired molded form such as preform, with pre-drying omitted.
2. Background Art
A pellet of the polyethylene terephthalate (PET) produced by bulk polymerizing of ethylene glycol with terephthalic acid or dimethyl terephthalate is an uncrystallized and transparent material. The PET whose moisture content does not reach an equilibrium has further moisture-absorption characteristics and thus when the PET is molten in a moisture absorbed condition, a hydrolysis develops to prevent the PET from being molded.
An uncrystallized PET pellet, when heated to a temperature above its glass transition point (Tg), becomes softened somewhat to have an adhesive characteristics. The adhesive characteristics causes the PET pellets to be poorly bitten into by a screw and thus not to be fed when molding. On the other hand, a crystallized PET pellet, even at a temperature above its Tg, is not softened, thereby keeping its hardness and having no adhesive characteristics.
Thus, material manufacturers dry an uncrystallized PET pellet at a temperature of 150.degree. to 160.degree. C. to produce a dehumidified and crystallized PET pellet which is sold as a commercially available molding material. This causes the ordinary and commercially available PET to become more expensive than an uncrystallized PET. The uncrystallized PET, which has been sold by some of material manufacturers, has to be dried to effect dehumidification and crystallization by molding manufacturers before use. Therefore, the crystallized PET described herein means a dried commercially available product, while the uncrystallized PET means an undried product. In molding a product using the PET as a material, even the crystallized PET has moisture-absorption characteristics and thus absorbs moisture to some extent varying with control conditions before it is used as a molding material; thus it is customary to dry preliminarily the PET immediately before molding.
It takes about four hours to accomplish the preliminary drying at a temperature of 150.degree. C. Both the increased usage of material per hour to improve molding cycle efficiency, and the prevention of a shortage of fed material during a long time operation causes a dryer installed to an injection molding machine to become inevitably large in size and the power consumption required for drying to tend to increase. Problems exist in that it takes a lot of time to make preparations before the molding operation, and that a malfunctioned dryer affects seriously the molding operation.
Injection molding means omitting the preliminary drying can be devised. As such molding means, there has been known a vent type molding machine in which an injection screw in a heating cylinder is divided into a first stage and a second stage, and the moisture content in a molten resin in the first stage is removed to the outside through a vent opening provided between both the stages.
In this well-known vent type molding machine, the screw groove of a feeding zone of the first stage leading from the rear end at which a hopper is located to a metering zone at the front end is formed in a deep groove; pellets of the material resin fed from the hopper in a humid condition are molten due to shearing heat and compression while being sequentially fed by screw rotation; and then the groove leads through the metering zone having a shallow groove to the second stage.
The screw groove in the second stage is formed in a deep groove, so that when the molten resin is transferred to the groove, the pressure of the molten resin is reduced due to the rapid increase in the groove depth of the screw to cause the moisture content in the resin to be vaporized, and the moisture content in the resin by screw rotation to be separated, which moisture content is then sucked and removed through the vent opening by a vacuum pump. The molten resin whose moisture content has been removed in this manner is fed by screw rotation to a metering zone at the front end of the second stage, and then to a position in front of the screw to a store, in a similar manner to an injection molding machine having an ordinary mechanism. The molten resin thus stored is injected and loaded into a cavity by the forward movement of the injection screw after stoppage of screw rotation.
Although generally the employment of such vent type injection unit may allow even a material having a large amount of moisture absorption to be molded without the preliminary drying, such unit is not applied to all resins, and with respect to the PET, particularly an uncrystallized one, the pellet is softened and compressed in a feed zone to cause it to be accumulated, whereby the PET in the hopper cannot enter the zone, and the biting by the screw becomes extremely poor such that the material cannot be fed. As a result, it has been quite difficult to perform injection molding without the necessity of the preliminary drying by employing a conventional vent type injection unit.